The rapidly increasing number of effects ascribed to cGMP in mammals include smooth muscle relaxation, platelet inhibition, neutrophil degranulation, vision, gene expression, ion and water transport, bone resorption, skin darkening, long-term nerve depression, and opiod action, cGMP mediates effects of natural signals such as nitric oxide, natriuretic peptides, and guanylins, as well as effects of medications such as nitroglycerin and sildenafil (Viagra/TM). Guanylyl cyclases catalyze synthesis of cGMP, and phosphodiesterases (PDE) catalyze cGMP degradation; the balance of these two activities determines the tissue level of cGMP. The known intracellular receptors that are believed to mediate cGMP effects are cGMP-dependent protein kinase (PKG), cGMP-gated ion channel proteins, cGMP-binding PDEs (PDE2, PDE5, PDE6, PDE 10, PDE11), and cAMP-dependent protein kinase (PKA) by cross-activation. The main subjects of this application are regulation of PKG and PDE5/PDE11, with particular emphasis on functional relationships between these two enzyme classes. It is suggested that up to six mechanisms exist by which PDE5 mediates decline of cGMP after PKG activation by cGMP elevation, which represents negative feedback regulation of the cGMP pathway. Some of these mechanisms increase PDE5 catalytic site affinity, so that elevation of cGMP by drugs such as Viagra TM would cause potentiation of their own effects. It is hypothesized that these mechanisms involve both allosteric regulation of PDE5 by cGMP binding as well as phosphorylation of PDE5 by PKG. The probability that prolonged cGMP elevation in tissues induces compensatory adjustment in levels of guanylyl cyclase, PDE5, and PKG will also be inspected. Investigation of the fundamental mechanisms by which PKG is autoinhibited, and by which autoinhibition is relieved will be carded out. Site-directed mutagenesis will be used to study the molecular roles of a conserved serine juxtaposed to the pseudosubstrate site of PKG, which contributes strongly to both autoinhibition and inhibition of cGMP binding. The roles of Arg- 59 in the pseudosubstrate site will also be scrutinized. Small angle x-ray scattering, 3D electron microscopy, and deuterium/hydrogen exchange will be used to examine the quaternary structure and domain topography of PKG, and to decipher the changes in these parameters produced by cGMP binding. Three potentially commercialized PDE5 inhibitors (sildenafil, vardenafil, tadalafil) have been labeled with tritium. They will be used to identify and quantify PDE5 and PDE11 in crude tissue extracts, and to search for other PDE inhibitor-binding proteins in these extracts. They will also be used to address unknown catalytic site features of PDE5 and PDE11 such as catalytic site heterogeneity, binding affinity, and effects of divalent cations on affinity. Effects of cGMP binding to allosteric sites (GAF domains) and effects of PDE5 phosphorylation by PKG on catalytic site affinity for radiolabeled PDE5 inhibitors will also be studied. A thorough physical and biochemical characterization of PDE11 will be carded out, and the likelihood of regulation of this enzyme by ligand binding to its GAF domains or by enzyme phosphorylation will be explored.